Light emitting diodes (LEDs) recently have become an indispensable lighting device due to their small size, fast lighting response and long life expectancy.
An LED is a diode (i.e. a semi-conductor element) and outputs light when appropriately energized. Generally, an LED emits light when subjected to a forward bias voltage greater than a threshold voltage (Vth) of the LED. The current rises sharply as the forward bias voltage increases beyond the threshold voltage (Vth). The brightness of the emitted light corresponds to the current through the LED.
However, in a general application, an alternating current (AC) LED device uses a current restrictor to restrict or limit current to a constant value and provides a constant and stable light output from the LED. Also, applying a constant current to the LED increases the LED lifetime.
With reference to FIG. 1, many patents about LED drivers exist. For example, U.S. Pat. No. 6,989,807, “LED driving device” discloses an “LED driving device (1)” that improves efficiency, power factor and power consumption by changing how many LEDs are lighted by an AC voltage. The “LED driving device (1)” comprises a power module (10), an LED array (13), multiple current controllers (11) and a voltage detector (12).
The LED array (13) comprises multiple LEDs connected in series. Each LED has an anode and a cathode.
The power module (10) is connected to an external power source and has a bridge rectifier. The external power source provides an alternating current (AC) power. The AC power is sinusoidal and has alternating negative and positive segments. The bridge rectifier inverts the negative segments of the AC to positive segments and forms a pulsating direct current (DC) voltage.
The LED array (13) is connected to the power module (10) and has multiple LEDs (131) connected in series.
The current controllers (11) are connected respectively to the cathodes of the LEDs (131).
The voltage detector (12) is connected to the power module (10) and the current controllers (11). The voltage detector (12) senses the pulsating DC voltage of the power module (10) and controls the current controllers (11) to turn the LEDs (131) ON or OFF based on the sensed pulsating DC voltage.
Accordingly, the LED driver selectively drives a certain number of LEDs at different voltage levels of the pulsating DC voltage. However, the LED driver is able to drive some LEDs at a very low AC voltage level. Those LEDs that have not been driven are idle, and the overall efficiency of the LED driver is reduced. Therefore, an efficient method of driving all LEDs for every AC voltage level is required.